The invention relates to an anti-skid apparatus as defined hereinafter. An anti-skid apparatus for a vehicle brake system of this kind is known from German Patent 31 47 149. Because its controllable check valve is disposed between the pressure reservoir and the master brake cylinder, the capacity of this pressure reservoir must suffice to hold such quantities of pressure fluid as must be withdrawn from the at least one wheel brake for approximately complete reduction of brake pressure generated in a panic braking situation. A pressure reservoir having this kind of capacity is large and therefore is heavy and expensive. If the pressure reservoir is not large enough, then there is the disadvantage that the return pump will generate impermissibly high pressures, if large quantities of pressure fluid must be withdrawn from the wheel brakes. German Patent 31 47 149 describes the usability of the generic anti-skid apparatus in two-tracked vehicles, such as passenger cars and one-tracked motor vehicles, such as motorcycles. The disposition of the controllable check valves has the effect that pressure oscillations occurring in anti-skid operation are kept away from the master brake cylinder and the brake pedal coupled with it.
German Offenlegungsschrift 31 07 963, to which the aforementioned German Patent 31 47 149 refers, discloses anti-skid apparatus for vehicle brake systems, each having one check valve that is located between the master brake cylinder and the anti-skid valve assembly and can be opened in the direction of this valve assembly, and alternatively has a pressure reservoir downstream of the outlet of the return pump, or a controllable pressure limiting valve between the outlet of the return pump and its inlet. The alternative having the pressure reservoir has the disadvantage already mentioned. The alternative having the controllable pressure limiting valve has the disadvantage that it requires a very large hydraulic vessel with a volumetric capacity equivalent to that of the pressure reservoir already described. Another disadvantage is that failure of the return pump causes the brake pedal to sag deeply in the event of anti-skid operation.
An anti-skid apparatus known from U.S. Pat. No. 4,703,979 has a pressure limiting device loaded by a spring assembly in the closing position, which is connected by its inlet to the outlet of the return pump and by its outlet to a main line that begins at the master brake cylinder and leads to anti-skid valve assemblies of the wheel brakes. The pressure of the pressure limiting valve assembly is in equilibrium with the pressure prevailing in the main line. The disadvantage is that because of the absence of a pressure reservoir that could be supplied from the return pump, a safety bypass valve is disposed between the outlet of the return pump and its inlet; this valve has a relatively high opening pressure and therefore places a large load on the return pump and entails major energy consumption. Moreover, this anti-skid apparatus is very complicated in general and is therefore especially expensive.